The death toll from air pollution, usually put at around 29,000 a year in the UK, could be substantially higher because the lethal effect of nitrogen dioxide (NO2), emitted during fossil fuel burning, has not been taken into account, experts believe.

Until now, only deaths linked to fine particles, less than 2.5micrometres in diameter (PM2.5) have been estimated. In cities, these come primarily from cars, lorries and buses but they are also produced by the burning of wood, heating oil or coal for domestic or industrial purposes, or in forest fires.

Deaths from air pollution globally are rising. The Global Burden of Disease study from the Institute of Health Metrics and Evaluation at the University of Washington in Seattle found that there were 3.4 million deaths from outdoor pollution in 2010, which was an increase from three million in 1990. Adding in the effects of indoor pollution, mostly from cooking fires in the developing world, brings the global death toll to seven million a year according to the World Health Organisation (WHO).

Policy director at NGO Transport and Environment Jos Dings said the solutions to the air pollution problem would also slash carbon emissions, creating a win-win for human health and the climate. “The more we move our transport system away from hydrocarbons – oil and biofuels – towards electricity, the better it is. This will simulteneously solve the air pollution and climate issues,” he said.

In addition, said Dings, the decline of coal and other fossil fuels, which are also sources of NO2, as sources of power in future would compound the health benefits of a shift to electric transport. “If you look into the future, the additional electricity that will come into the system over the next decades is likely to be very renewable. Three-quarters of the additional capacity that we currently install in Europe in the power sector is wind and solar. The trend is very much in the good direction,” he said.

In Europe, the WHO estimates about 500,000 people die prematurely as a result of air pollution every year.

But this death toll may be significantly higher if NO2 were factored in.

Adding those deaths to the count is difficult though, because there is an overlap. It is hard to know which traffic emissions have done harm to people who live alongside major roads. But a WHO report last year concluded that NO2 was an additional cause of death and the UK government’s committee on the medical effects of air pollution (COMEAP) is now attempting to put figures on it.

Dr Heather Walton, senior lecturer in environmental health at King’s College London, said that the relative risks of PM2.5 and NO2 were similar. In the UK, scientists work on a 6% rise in mortality for a 10 microgramme per cubic metre increase in the PM2.5 concentration. The WHO’s health risks of air pollution in Europe (HRAPIE) report put the equivalent mortality increase for NO2 at 5.5%.

But annual average concentrations of NO2 are higher than those for PM2.5. “So, on the face of it, you might expect to get a bigger number for NO2 than for PM2.5. However, it is not as straightforward as that. For one thing, as more polluted places usually have higher levels of both pollutants and both affect mortality, it can be difficult to separate the effects,” said Walton.

The WHO report said the overlap could be as much as 33%. It also recommended only calculating the impact of NO2 when concentrations exceed 20 microgrammes per cubic metre.

Discussions are now taking place in COMEAP and beyond on how to add in the effects of NO2 on deaths from air pollution, but even though, Walton says, this is “certainly one of the key issues in the field at the moment”, no final number is expected any time soon.

Much depends on where you live with the highest risks close to main roads

The link between air pollution and adverse health consequences is becoming clearer but it was only in the early 1990s that strong research evidence began to be published making the connection. In 1993, Douglas Dockery from the Harvard School of Public Health and colleagues published a study of over 8,000 adults in six US cities suggesting higher levels of fine particles in the air was associated with higher death rates - once smoking was excluded. In 2009, another seminal paper from the same group showed that cleaning up air pollution increased life expectancy.

But much depends on where you live – it is not which city that matters, but where in the city, with the highest risks close to main roads. Walking or cycling along fume-filled streets is clearly an issue, but not as big an issue as living on them. A small study of 50 schoolchildren by Mark Nieuwenhuijsen, a professor at Barcelona’s Center for Research in Environmental Epidemiology demonstrated this year that their exposure was higher when walking back from school, but it was during the 15 hours a day in the home that their exposure to air pollutants was greatest.

This was part of a bigger study looking at the effects of air pollution on children’s cognitive development. This month the group published their paper in PLoS Medicine, showing that children in Barcelona at schools where air pollution is heavier did less well over time in memory and attention tests than their peers elsewhere.

The effects on children are of increasing concern. The latest results from a two decade-long study in Los Angeles, published in the prestigious New England Journal of Medicine last month provided some of the most convincing evidence yet of the effect of air pollution on the growth of children’s lungs.

The paper offered some good news. Levels of NO2 and PM2.5 have dropped by 40% in the urban areas where the children in the study live. James Gauderman, professor of preventive medicine at the Keck School of Medicine of the University of Southern California (USC) and his team, who have run the Children’s Health Study for two decades and who first identified a link between air pollution and impaired children’s lung function, report that as a result of the clean-up, today’s children can breathe more easily. In the original 1994-98 cohort of children, 8% had impaired lung function, which often leads in adulthood to respiratory disease and also heart disease. In the 2007-11 cohort, that had dropped to 3.6%.

The research is a breakthrough because of the strength of the evidence it provides that fumes and particles are a cause of lung damage.

“It’s really, really important, because when you talk about associations with lung growth people say well it’s an association – you haven’t proven there is a link,” says Ian Mudway, lecturer in respiratory toxicology at King’s College London. “But if you then show that actually you can address the deficit by reducing air pollution you go beyond the point of saying it’s an association. You begin to have something which is a much more causal relationship.”

In October 2013, ESCAPE reported on air pollution and low birthweight babies. Among the participating cities was Bradford in England. As elsewhere, in the Born in Bradford cohort of mothers and babies, there was a higher likelihood of restricted foetal growth if the woman in her pregnancy was living in an area of high air pollution. Low birthweight babies often have health problems in later life.

Studies have also shown reduced fertility in areas with high air pollution and links to obesity. There is concern now about cognitive effects as well, with research looking at dementia and neurological degeneration.

• Additional reporting by Karl Mathiesen

• This article was amended on 31 January 2017. An earlier version referred to deaths linked to fine particles less than 2.5mm in diameter. That has been corrected to 2.5 micrometres.